In recent years, an excimer laser for generating light belonging to ultraviolet frequency range has been used as a light source of light exposure in photolithography.
The excimer laser will be explained by referring to, for example, a product manufactured by Lambda Physik Company. FIG. 16 shows a laser of an injection lock type which comprises a resonator having a total reflective mirror 11 and a beam output mirror 12, an oscillator part 10 having a dispersion prism 13, apertures 14 and 15, and a pair of electrodes 16, and amplifier part 20 optically coupled to the oscillator part 10 through mirrors 17 an 18 and having mirrors 21 and 22 and a pair of electrodes 23.
In this laser, the oscillator part 10 functions to divide wavelengths of light waves through the dispersing prism (or grating, etalon or the like) 13 and to converge into a beam through the apertures 14 and 15, whereby such a laser beam can be obtained that has a narrow spectrum linewidth and coherent beam characteristics. The laser beam thus obtained is synchronously injected into the amplifier part 20 which forms an unstable resonator, to forcedly oscillate in its cavity mode and increase its power.
Such an arrangement as shown in FIG. 17 is published from the American Telephone and Telegraph Company (AT&T) in Proceedings of SPIE(conference name), "MICRO DEVICE CONFERENCE" by Victor Pol et al., issued April, 1986. With this laser, an exciting zone 34 is provided within a laser resonator having a total reflective mirror 31 and a beam output mirror 32, and two etalons 33 are disposed between the exciting zone 34 and beam output mirror 32, thus enabling oscillation of the laser having a narrow spectrum linewidth and a limited number of transversal modes.
However, such an excimer laser of the injection lock type has been defective in that it can provide a large output laser beam having a narrow spectrum linewidth but also of a single-mode (coherent beam), which results in that when it is used as a light source for projection reduction, it generates a speckle pattern disabling acquisition of a high resolution.
Such an injection lock type of laser has also had a problem that the use of only the oscillator part 10 using the apertures 14 and 15 causes a remarkable reduction of laser output, which requires the provision of the amplifier part 20 for synchronous injection, so that, if it is impossible to put the laser in synchronism through highly frequent repetition, it spontaneously oscillates, thus making large the size of the laser device itself.
Further, the AT&T laser also generates a speckle pattern because the number of transversal modes becomes insufficient when it is employed in an ordinary illuminating optical system for light exposure. For this reason, the illuminating engineering system must be of a scan mirror type, which involves the complicated arrangement and control of the light exposure apparatus. In addition, this type of laser, which is small in its power, requires a light exposure time of 25 seconds, leading practically to a limited range of applications.
A primary object of the present invention is, therefore, to provide a multi-mode, narrow-band oscillation, excimer laser which eliminates the above defects in the prior art and which can produce a laser beam having a narrow frequency band, a high output and a spatially incoherent beam characteristic and suitable for reduction projection.